In a cryptographic system that provides to a user processing for enciphering (encrypting) plaintext and processing for deciphering (decrypting) ciphertext using a computer, a particular user may, in some cases, be given authorization to decipher ciphertext (decryption authorization) created by another user. In this case, in such a manner that the other user merely distributes a key used for enciphering/deciphering processing to the particular user, the change in the contents of the authorization cannot be quickly coped with. Further, in a cryptographic system utilizing a network, measures to prevent the action of impersonating a user having decryption authorization to improperly decipher ciphertext created by another user are also required.